Field of the Invention
This invention relates to an improved mechanism for assembling cutters on supporting bearing spindles in roller cutter earth boring bits. The invention reduces the number of catastrophic drill bit failures due to cutter loss during the drilling operation, thereby providing greater reliability and reduced wellbore drilling costs.
Much of the cost associated with drilling wellbores into the earth for recovery of hydrocarbons is directly related to the performance of the earth boring bit. Although the cost of the bit itself is often trivial in relation to other drilling expenses, the consequences of an unexpected or premature bit failure are significant. For example, a typical 77/8" three rolling cutter drill bit costs about $4,500. If one or more rolling cutters of this drill bit were to come off during drilling, the additional cost to recover the "junk" cutter could exceed $150,000. If the wellbore is deeper than about 10,000 ft. or is deviated from vertical more than about 30 degrees (as in directional drilling), the cost to recover the lost cutter could easily be $300,000. Therefore, one critical component of all modern, sealed rolling cutter earth boring bits is its mechanism for retaining the cutter on the body of the bit.
Cutter retention systems are well known in the art. For example, ball bearings can be inserted through a hole in the body to fill a groove between the rolling cutter and the bit body as shown in U.S. Pat. No. 3,989,314. Alternatively, a snap ring can be positioned in the same general area as the ball bearings as shown in U.S. Pat. No. 4,236,764. Finally, a split threaded thrust bearing member can be installed in the bit as shown in U.S. Pat. No. 3,971,600. Other threaded ring rolling cutter retention mechanisms are shown in U.S. Pat. Nos. 4,911,255; 4,991,671; 5,012,701 and 5,024,539.
Through several years of wide commercial use, the threaded ring retention mechanism has been found to provide superior cutter retention performance as compared to the other retention systems as long as the threaded ring remains securely seated within the rolling cutter. If the threaded ring becomes unseated, i.e. unscrewed from its intended position, the resulting excessive axial cutter displacement is detrimental to the cutter seal, resulting in premature bearing failure and shorter than expected bit life. As described in U.S. Pat. No. 3,971,600, the threaded ring is designed to resist unseating (unscrewing) after the bit is assembled by provision of a right hand thread so that the threaded ring will not loosen as the cutter turns in drilling using the normal clockwise drill bit rotation. In spite of this design, extensive analysis of used bits has indicated that the most common failure mode of the threaded ring was unseating.
Forces present during drilling can cause loosening of the threaded ring, especially when drilling wellbores with angular deviations greater than about 30 degrees from vertical. There are times during the drilling operation when the rolling cutter of a drill bit experiences reverse rotation. Because the threaded ring is designed to tighten during normal cutter rotation, reverse rotation of the cutter can cause the threaded ring to loosen. During normal wellbore drilling operations the rotation of the bit is often stopped so that an additional section of drill pipe can be added to the drill string. During this operation, the drill string and drill bit are first raised then lowered in the wellbore without rotation. If one cutter of the bit is scraping the side of the wellbore as the drill string is raised or lowered, the dragging action can cause reverse rotation of the cutter. The force of gravity urging the bit against the side of a directional wellbore makes this sidewall scraping worse, and consequently increases the chances of reverse cutter rotation, as the wellbore increasingly deviates from vertical.
The trend in today's oil and gas industry is to drill more highly deviated and horizontal wellbores. Due to the higher hourly rates for the specialised equipment required for this drilling, bit performance and reliability are critical. This is also the same type of drilling where reverse cutter rotation is most likely to occur. It is critical to keep the threaded retention ring securely locked in place during any conditions which may be encountered during drilling. It is also critical to provide very high resistance to further turning of the ring if it were to become loose. Therefore, the reliability of the locking means for the threaded retention ring is critical. For these reasons, there is a need for a drill bit with a threaded ring cutter retention system which is capable of withstanding reverse cutter rotation or any other condition encountered during drilling without loosening of the threaded ring.